Method and apparatus for transmitting frame on basis of sounding procedure

ABSTRACT

A method and apparatus for transmitting a frame on the basis of a sounding procedure is disclosed. The method for transmitting the frame on the basis of a sounding procedure in a wireless LAN may comprise the steps of: transmitting an NDPA frame to each of multiple STAs by an AP wherein the NDPA frame notifies of the transmission of NDP; transmitting the NDP to each of the multiple STAs by the AP; receiving, by the AP, multiple feedback frames transmitted through multiple transmission sources allocated to the multiple STAs in time sources overlapped by the multiple STAs, respectively; and transmitting, by the AP, multiple downlink frames to the multiple STAs respectively.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for transmitting a frame on thebasis of a sounding procedure.

Related Art

A wireless local area network (WLAN) system may support a downlink multiuser-multiple input multiple output (MU-MIMO) transmission method for ahigher throughput. An access point (AP) may simultaneously transmit dataframes to at least one or more stations (STAs) which are MIMO-pairedthrough a downlink. In the legacy WLAN system, the maximum number ofpaired STAs may be 4, and when the maximum number of spatial streams is8, up to 4 spatial streams may be allocated to each STA.

For example, in the WLAN system, the AP may simultaneously transmit datato an STA group including at least one or more STAs among a plurality ofSTAs associated with the AP. In a WLAN system in which the AP not onlyperforms MU-MIMO transmission to the STAs but also supports a tunneleddirect link setup (TDLS) or direct link setup (DLS) and a mesh network,an STA which intends to transmit data may transmit a PLCP protocol dataunit (PPDU) to the plurality of STAs by using the MU-MIMO transmissionscheme. In case of using MU-MIMO, data to be transmitted to each STA maybe transmitted through a different spatial stream.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of receiving aframe on the basis of a sounding procedure.

Another object of the present invention is to provide an apparatus forperforming a method of receiving a frame on the basis of a soundingprocedure.

In order to achieve the above object of the present invention, accordingto one aspect of the present invention, a method of transmitting a frameon the basis of a sounding procedure in a wireless local area network(WLAN) may include: transmitting by an access point (AP) a null datapacket announcement (NDPA) frame to each of a plurality of stations(STAs), wherein the NDPA frame reports transmission of a null datapacket (NDP); transmitting by the AP the NDP to each of the plurality ofSTAs; receiving by the AP a plurality of feedback frames to betransmitted respectively through a plurality of transmission resourcesallocated respectively to the plurality of STAs on an overlapping timeresource respectively by the plurality of STAs; and transmitting by theAP a plurality of downlink frames respectively to the plurality of STAs.The plurality of feedback frames may respectively include a plurality ofpieces of channel state information determined on the basis of the NDP.The plurality of downlink frames may be respectively transmitted basedon the plurality of pieces of channel state information within aspecific time after respectively receiving the plurality of feedbackframes.

In order to achieve the above object of the present invention, accordingto another aspect of the present invention, an AP for transmitting aframe on the basis of a sounding procedure in a WLAN may include: aradio frequency (RF) unit implemented to transmit or receive a radiosignal; and a processor operatively coupled to the RF unit. Theprocessor may be configured for: transmitting an NDPA frame to each of aplurality of STAs, wherein the NDPA frame reports transmission of anNDP; transmitting the NDP to each of the plurality of STAs; receiving aplurality of feedback frames to be transmitted respectively through aplurality of transmission resources allocated respectively to theplurality of STAs on an overlapping time resource respectively by theplurality of STAs; and transmitting a plurality of downlink framesrespectively to the plurality of STAs. The plurality of feedback framesmay respectively include a plurality of pieces of channel stateinformation determined on the basis of the NDP. The plurality ofdownlink frames may be respectively transmitted based on the pluralityof pieces of channel state information within a specific time afterrespectively receiving the plurality of feedback frames.

Since a frame is transmitted based on channel state information acquiredon the basis of a sounding procedure, data transmission efficiency canbe increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a concept view illustrating the structure of a wireless localarea network (WLAN).

FIG. 2 is a conceptual view showing an interval (or gap) between frames.

FIG. 3 shows a measurement of a signal to interference plus noise ratio(SINR) of an uplink frame.

FIG. 4 is a concept view illustrating a method of transmitting a frameon the basis of a sounding procedure according to an embodiment of thepresent invention.

FIG. 5 is a concept view illustrating a format of a null data packetannouncement (NDPA) PLCP protocol data unit (PPDU) according to anembodiment of the present invention.

FIG. 6 is a concept view illustrating a null data packet (NDP) accordingto an embodiment of the present invention.

FIG. 7 is a concept view illustrating a feedback PPDU according to anembodiment of the present invention.

FIG. 8 is a concept view illustrating a method of transmitting a frameon the basis of a sounding procedure according to an embodiment of thepresent invention.

FIG. 9 is a concept view illustrating a method of transmitting a frameon the basis of a sounding procedure according to an embodiment of thepresent invention.

FIG. 10 is a concept view illustrating a method of transmitting a frameon the basis of a sounding procedure according to an embodiment of thepresent invention.

FIG. 11 is a concept view illustrating an NDPA PPDU format according toan embodiment of the present invention.

FIG. 12 is a concept view illustrating an NDP according to an embodimentof the present invention.

FIG. 13 is a concept view illustrating a PPDU format for a soundingprocedure according to an embodiment of the present invention.

FIG. 14 is a concept view illustrating a PPDU format for carrying anacknowledgment (ACK) frame according to an embodiment of the presentinvention.

FIG. 15 is a block diagram of a wireless device according to anembodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a concept view illustrating the structure of a wireless localarea network (WLAN).

An upper part of FIG. 1 shows the structure of the IEEE (institute ofelectrical and electronic engineers) 802.11 infrastructure network.

Referring to the upper part of FIG. 1, the WLAN system may include oneor more basic service sets (BSSs, 100 and 105). The BSS 100 or 105 is aset of an AP such as AP (access point) 125 and an STA such as STA1(station) 100-1 that may successfully sync with each other tocommunicate with each other and is not the concept to indicate aparticular area. The BSS 105 may include one AP 130 and one or more STAs105-1 and 105-2 connectable to the AP 130.

The infrastructure BSS may include at least one STA, APs 125 and 130providing a distribution service, and a distribution system (DS) 110connecting multiple APs.

The distribution system 110 may implement an extended service set (ESS)140 by connecting a number of BSSs 100 and 105. The ESS 140 may be usedas a term to denote one network configured of one or more APs 125 and130 connected via the distribution system 110. The APs included in oneESS 140 may have the same SSID (service set identification).

The portal 120 may function as a bridge that performs connection of theWLAN network (IEEE 802.11) with other network (for example, 802.X).

In the infrastructure network as shown in the upper part of FIG. 1, anetwork between the APs 125 and 130 and a network between the APs 125and 130 and the STAs 100-1, 105-1, and 105-2 may be implemented.However, without the APs 125 and 130, a network may be establishedbetween the STAs to perform communication. The network that isestablished between the STAs without the APs 125 and 130 to performcommunication is defined as an ad-hoc network or an independent BSS(basic service set).

A lower part of FIG. 1 is a concept view illustrating an independentBSS.

Referring to the lower part of FIG. 1, the independent BSS (IBSS) is aBSS operating in ad-hoc mode. The IBSS does not include an AP, so thatit lacks a centralized management entity. In other words, in the IBSS,the STAs 150-1, 150-2, 150-3, 155-4 and 155-5 are managed in adistributed manner. In the IBSS, all of the STAs 150-1, 150-2, 150-3,155-4 and 155-5 may be mobile STAs, and access to the distributionsystem is not allowed so that the IBSS forms a self-contained network.

The STA is some functional medium that includes a medium access control(MAC) following the IEEE (Institute of Electrical and ElectronicsEngineers) 802.11 standards and that includes a physical layer interfacefor radio media, and the term “STA” may, in its definition, include bothan AP and a non-AP STA (station).

The STA may be referred to by various terms such as mobile terminal,wireless device, wireless transmit/receive unit (WTRU), user equipment(UE), mobile station (MS), mobile subscriber unit, or simply referred toas a user.

FIG. 2 is a conceptual view showing an interval (or gap) between frames.

Referring to FIG. 2, a time difference between two frames beingtransmitted within a medium may be referred to as an IFS (interframespace). The priority of an STA occupying a wireless medium (or radiomedium) may be decided based on IFSs having different lengths. Framesbeing transmitted within a medium may be transmitted based on IFSshaving different lengths. For example, IFSs having different lengths maybe used for transmitting frames within the medium.

(1) SIFS (short inter frame symbol): This is used for the transmissionof RTS (request to send) frames/CTS (clear to send) frames, ACK(acknowledgement) frames.

(2) PIFS (PCF (point coordination function) IFS): This is used for thetransmission of PCF frames (e.g., channel switch announcement frames,TIM (Traffic indication map) frames, and so on) and for the frametransmission of STAs performing channel access based on PCF (pointcoordination function).

(3) DIFS (DCF IFS): This is used for frame transmission of STAsperforming DCF based channel access.

(4) EIFS (extended IFS): This is used only when a frame transmissionerror occurs, and this is not a fixed gap (or interval).

The calculation equation for each IFS is as shown below in Equation 1 toEquation 3, and the numerical values given in the parentheses next toeach parameter may correspond to general numerical values respective toeach parameter. The value of each parameter may vary in accordance withthe capacity of the STA or in accordance with the communicationenvironment. Each equation is disclosed in 9.3.7 DCF timing relation ofthe IEEE P802.11-REVmcTM/D2.0 document, which is disclosed in October2013, and the parameters used in each equation are disclosed in 6.5.4PLME-CHARACTERISTICS.confirm of the IEEE P802.11-REVmcTM/D2.0 documentand 6.5 PLME SAP interface of the IEEE Std 802.11ac™-2013 document, andso on.

SIFS(16 μs)=aRxRFDelay(0.5)+aRxPLCPDelay(12.5)+aMACProcessingDelay(1 or<2)+aRxTxTurnaroundTime(<2)

aRxTxTurnaroundTime=aTxPLCPDelay(1)+aRxTxSwitchTime(0.25)+aTxRampOnTime(0.25)+aTxRFDelay(0.5)  <Equation 1>

Referring to FIG. 1, the SIFS may correspond to a value that is based ona RF (radio frequency) delay, a PLCP (physical layer convergenceprotocol) delay, MAC (medium access control) processing delay, andshifting time from Rx to Tx. For example, the SIFS may correspond to atime period starting from a time when the last symbol of a receivingframe of the STA is received from a medium (or air interface) up to atime when a first symbol of a transmitting frame of the STA istransmitted to the medium (or air interface).

PIFS(25 μs)=aSIFSTime+aSlotTime

aSlotTime=aCCATime(<4)+aRxTxTurnaroundTime(<2)+aAirPropagationTime(<1)+aMACProcessingDelay(<2)  <Equation2>

Herein, aAirProgationTime may correspond to two times the propagationtime (or propagation delay) for signal transmission at a maximumdistance between STAs that are synchronized to the furthermost slotswithin a communicatable range. Herein, the aAirProgationTime maycorrespond to a value that is equal to or less than 1 μs. The radio wavemay be propagated at 300 m/μs

DIFS(distributed (coordination function) interframe space)(34μs)=aSIFSTime+2×aSlotTime  <Equation 3>

Referring to Equation 1 to Equation 3, the SIFS, PIFS, and DIFS valuesmay vary in accordance with the capability of the STAs and/or inaccordance with the wireless communication environment. For example, inaccordance with the capability of the STAs and/or in accordance with thewireless communication environment, the SIFS may be given a maximumvalue of 16 μs, and the PIFS may be given a value ranging from a minimumvalue of 16 μs to a maximum value of 25 μs.

An access point (AP) operating in a wireless local area network (WLAN)system may transmit data through the same time resource to each of aplurality of stations (STAs). Such transmission of the AP may beexpressed by the term ‘downlink multi-user (DL MU) transmission’ whentransmission from the AP to the STA is defined as downlink transmission.

In the legacy WLAN system, the AP may perform DL MU transmission on thebasis of MU MIMO, and such transmission may be expressed by the term ‘DLMU MIMO transmission’. In the embodiment of the present invention, theAP may perform DL MU transmission on the basis of OFDMA, and suchtransmission may be expressed by the term ‘DL MU OFDMA transmission’. Incase of using the DL MU OFDMA transmission, the AP may transmit adownlink frame to each of a plurality of STAs respectively through aplurality of frequency resources on an overlapping time resource.

A PPDU, frame, and data transmitted through downlink transmission may berespectively expressed by the term ‘downlink PPDU’, ‘downlink frame’,and ‘downlink data’. The PPDU may be a data unit including a PPDU headerand a physical layer service data unit (PDSU) (or a MAC protocol dataunit (MPDU)). The PPDU header may include a PHY header and a PHYpreamble, and the PDSU (or MPDU) may include a frame or may indicate theframe.

On the contrary, transmission from the STA to the AP may be regarded asuplink transmission, and data transmission from the plurality of STAs tothe AP on the same time resource may be expressed by the term ‘uplinkmulti-user (UL MU) transmission’. Unlike in the legacy WLAN system, theUL MU transmission may be supported in a WLAN system according to theembodiment of the present invention. A PPDU, frame, and data transmittedthrough uplink transmission may be respectively expressed by the term‘uplink PPDU’, ‘uplink frame’, and ‘uplink data’. Uplink transmissionperformed by each of the plurality of STAs may be performed on afrequency domain or a spatial domain.

If the uplink transmission performed by each of the plurality of STAs isperformed on the frequency domain, different frequencies may beallocated respectively for the plurality of STAs as an uplinktransmission resource on the basis of orthogonal frequency divisionmultiplexing (OFDMA). The plurality of STAs may transmit an uplink frameto the AP respectively through different frequency resources. Such atransmission method using the different frequency resource may beexpressed by the term ‘UL MU OFDMA transmission method’.

If the uplink transmission performed by each of the plurality of STAs isperformed on the spatial domain, different spatial streams may berespectively allocated for the plurality of STAs, and thus the STAs maytransmit an uplink frame to the AP respectively through differentspatial streams. Such a transmission method using the different spatialresource may be expressed by the term ‘UL MU MIMO transmission method’.

FIG. 3 shows a measurement of an SINR of an uplink frame.

Disclosed in FIG. 3 is a signal to interference plus noise ratio (SINR)measured in an access point (AP) when an STA transmits an uplink frameon the basis of a space time block code (STBC).

Referring to FIG. 3, an X-axis represents an experimental time from 2000ms to 5000 ms, and a Y-axis represents an SINR based on a time.Referring to the graph, it can be seen that an SINR value is changedsignificantly whenever each frame is transmitted. In this case, it maybe difficult to determine a parameter (a modulation and coding scheme(MCS)) based on a channel state for current frame transmission on thebasis of SINR information of a previous feedback frame.

In addition, a result of performing an experiment by setting feedbackinformation (SINR) as a factor in an outdoor large BSS scenario is asfollows.

A first case may be a case where there is no feedback for an SINR. Inthis case, an STA may perform cyclic redundancy check (CRC) startingfrom a modulation and coding scheme (MCS) level (or index) 0, and mayincrease an MCS level (or index) by 1 if transmission is successful 10times, and may decrease the MCS level by 1 if transmission fails twotimes.

A second case may be a case where there is a feedback for an SINR butfeedback information is determined on TXOP of a previous frame (or aframe transmitted in a range outside a threshold time with respect to acurrent frame) (feedback (previous)). In this case, an STA may determinechannel variation information (e.g., MCS) determined depending onfeedback information (or channel state information) by subtracting oradding an SINR margin (or 0.5 dB, 2 dB) from or to a current SINRaccording to CRC check information of a previous frame and/or whether toreceive an ACK frame.

A third case may be a case where there is a feedback for an SINR butfeedback information is determined on TXOP of a current frame (or aframe transmitted in a range within a specific threshold time withrespect to the current frame) (feedback (current)). For example, the STAmay generate feedback information on the basis of an NDPA frame and NDPreceived from an AP within a threshold time (e.g., SIFS time) withrespect to the current frame. For another example, the feedbackinformation may be generated through a frame acquired on TXOP configuredbased on a procedure of transmitting and receiving an RTS frame and aCTS frame. The STA may transmit the generated feedback information(e.g., SINR information) to the AP. The AP may determine channelvariation information on the basis of the feedback information, and maygenerate a downlink frame on the basis of the determined channelvariation information and transmit it to the STA. The AP may determinethe channel variation information (e.g., MCS) by subtracting or addingan SINR margin (e.g., 0.5 dB, 2 dB) from or to the current SINRaccording to the received feedback information and/or whether thefeedback frame is received. The determined MCS may be used by the AP totransmit a next downlink frame.

Table 1 below shows a downlink throughput and an uplink throughput whenthe feedback information is not received, for a case where the feedbackinformation is acquired on TXOP of a previous frame and for a case wherethe feedback information is acquired on TXOP of a current frame.

TABLE 1 Experi- DL UL mental Tput MCS 0/1/ . . . /8 Tput UL MCS 0/1/ . .. /8 option (Mbps) distribution (%) (Mbps) distribution (%) No 8.465/5/4/6/7/6/8/11/48 11.0 32/13/14/13/13/7/3/ feedback 2/2 Feedback 8.166/3/1/8/3/5/3/11/59 10.0 42/5/11/14/9/6/7/6/0 (previous) Feedback 10.09/2/3/5/4/3/3/11/61 18.5 42/5/6/9/8/4/3/12/12 (current)

Referring to Table 1, it can be seen that the case where the samefeedback information is determined on the TXOP of the current frame (orthe frame transmitted in the range within the specific threshold timewith respect to the current frame) (i.e., feedback (previous)) is notdifferent from the no-feedback case, or disadvantageously, hasperformance deterioration due to an overhead.

The case where the feedback information is determined on the TXOP of thecurrent frame (or the frame transmitted in the range within the specificthreshold time with respect to the current frame) has relatively higherperformance in terms of a downlink throughput and an uplink throughputin comparison with the no-feedback case and the feedback (previous)case.

A procedure of acquiring feedback information on the basis of a soundingprocedure using an NDPA frame is defined in the legacy WLAN system.However, a transmission timing of a frame based on the acquired feedbackinformation is not defined. In the legacy WLAN system, an STA performschannel access in a contention based manner to transmit the frame on thebasis of the feedback information. In this case, an interval between atiming of acquiring the feedback information of the STA and a timing oftransmitting the frame based on the feedback information of the STA maybe increased. The increase in the interval between the timings may causeperformance deterioration as shown in the aforementioned Table 1.

In addition, in order to transmit feedback information and to transmit adownlink frame based on the feedback information on the basis of DL MUtransmission and UL MU transmission according to the embodiment of thepresent invention, a grouping procedure may be necessary for a pluralityof STAs for receiving a downlink frame based on DL MU transmission andfor transmitting an uplink frame based on UL MU transmission.

If the grouping procedure for the plurality of STAs, the procedure ofacquiring the feedback information from the plurality of STAs after thegrouping of the plurality of STAs, and the procedure for transmitting aframe based on the feedback information after acquiring the feedbackinformation from the plurality of STAs are performed at differenttimings, there may be not much significant advantage based on thefeedback information.

Accordingly, in the embodiment of the present invention, the STA maydetermine channel variation information (MCS, the number of streams, abeamforming vector, etc.) on the basis of the feedback informationincluded in the feedback frame transmitted in an adjacent time, and maytransmit the frame. Hereinafter, for convenience of explanation, theembodiment of the present invention discloses a method in which the APacquires the feedback information from the STA, and transmits a downlinkframe (e.g., a downlink data frame, a downlink management frame) on thebasis of the acquired feedback information.

However, hereinafter, the AP may be interpreted as the STA according toa situation (e.g., a WLAN system supporting a tunneled direct link setup(TDLS) or direct link setup (DLS) and a mesh network) in the embodimentof the present invention. In addition, on the contrary, even if the STAacquires feedback information and transmits an uplink frame based on theacquired feedback information, a procedure of transmitting/receiving adata unit (e.g., PPDU or a frame) on the basis of a sounding procedureaccording to the embodiment of the present invention may also beapplied.

FIG. 4 is a concept view illustrating a method of transmitting a frameon the basis of a sounding procedure according to an embodiment of thepresent invention.

Disclosed in FIG. 4 is a method of transmitting a frame on the basis ofa sounding procedure for a single STA. The sounding procedure may be aprocedure for acquiring channel state information. More specifically,disclosed is a method in which an AP performs the sounding procedurewith respect to the STA, and transmits a downlink frame to the STAthrough SU-MIMO (or OFDMA) according to feedback information acquired onthe basis of the sounding procedure.

The AP may transmit a null data packet announcement (NDPA) frame 400 tothe STA for the sounding procedure, and may transmit an NDP 410 after aspecific time (e.g., a short interframe space (SIFS)).

The NDPA frame 400 may be used to report to the STA that the soundingprocedure starts and the NDP 410 is transmitted. The NDPA frame 400 mayinclude an STA information field. The STA information field may indicatean STA for receiving the NDP 410 to be transmitted after the NDPA frame400 and for transmitting a feedback frame. The STA indicated on thebasis of the STA information field may estimate a channel on the basisof the NDP 410 and may transmit a feedback frame 420 including thechannel state information to the AP. That is, the STA may determinewhether to transmit the feedback frame 420 to the AP by participating inchannel sounding on the basis of the STA information field included inthe received NDPA frame 400.

The NDP 410 may have a format which includes only a PPDU header byomitting a data field from the typical PPDU. The NDP 410 may be precodedby the AP on the basis of a specific precoding matrix. Upon receivingthe NDP 410, the STA may estimate a channel on the basis of a trainingfield (e.g., HE-LTF) of the NDP 410 and may acquire channel stateinformation. Since the NDP 410 does not have the data field, lengthinformation indicating a PSDU length included in the data field of theNDP 410 or a length of an aggregate-MAC protocol data unit (A-MPDU)included in the PSDU may be set to 0.

Each of the NDPA frame 400 and the NDP 410 may be transmitted through afull bandwidth for transmission of the NDPA frame 400 and the NDP 410.The PPDU format may be expressed by the term ‘non-duplicated PPDUformat’.

Alternatively, each of the NDPA frame 400 and the NDP 410 may betransmitted through a plurality of channels on the basis of a duplicatePPDU format. The duplicate PPDU format may be transmitted through abandwidth greater than 20 MHz (e.g., 40 MHz, 80 MHz, 160 MHz, 80 MHz+80MHz, etc.) by replicating the PPDU format transmitted through anadjacent channel (or a primary channel) (20 MHz). When a duplicateformat 450 is used, the same data may be transmitted through each of aplurality of channels (a replication target channel and a replicationchannel). That is, an NDPA PPDU (or NDP) may be used based on aduplicate format used to carry replicated information through each of aplurality of channels. The NDPA frame 400 and the NDP 410 having thenon-duplicate PPDU format or the duplicate PPDU format may betransmitted to at least one STA through at least one space-time stream.If the AP receives a feedback frame from a single STA as shown in FIG.4, the AP may transmit the NDPA frame 400 and the NDP 410 to the singleSTA through at least one stream. If the AP receives the feedback framefrom the single STA, the NDPA frame 400 may indicate at least one STAfor transmitting the feedback frame. An LTF may be transmitted throughat least one space-time stream through the NDP 410, and the STA maytransmit to the AP a feedback frame including channel state informationmeasured on the basis of the LTF of an indicated space-time stream andan indicated frequency domain.

Alternatively, the NDPA frame 400 and the NDP 410 may be transmittedbased on a DL MU transmission method. More specifically, the NDPA frame400 and the NDP 410 may be transmitted to a plurality of STAs throughdifferent space-time streams on the basis of DL MU MIMO transmission, ormay be transmitted to the plurality of STAs through different frequencyresources (or a subband, a channel) on the basis of DL MU OFDMA. In thiscase, the NDPA frame 400 and the NDP 410 transmitted through thedifferent space-time streams or the different frequency resources mayinclude different information. That is, the AP may transmit a pluralityof NDPA frames respectively to the plurality of STAs, and may transmit aplurality of NDPs respectively to the plurality of STAs. For example,the NDPA frame transmitted through a specific space-time stream or aspecific frequency resource may indicate only a specific STA fortransmitting a feedback frame.

Hereinafter, an embodiment of the present invention is described forconvenience of explanation by assuming a case where an NDPA frame and anNDP are transmitted to at least one STA through at least one stream onthe basis of a non-duplicate PPDU format or a duplicate PPDU format.

The STA may perform channel estimation on the basis of the NDP 410, andmay transmit acquired channel state information to the AP through thefeedback frame 420. A channel bandwidth used for transmission of thefeedback frame 420 may be set to be narrower than or equal to a channelbandwidth used for transmission of the NDPA frame 400. The feedbackframe 420 may include channel state information (or stream stateinformation) for each of an indicated space-time stream (or spatialstream).

If the NDP 410 is not transmitted based on beamforming, the feedbackframe 420 may include a high throughput (HT) control field and a channelinformation control field (e.g., very high throughput (VHT) MIMO controlfield or an HE MU control field). The HT control field may includeinformation regarding Nsts (the number of space-time streams), MCS,bandwidth (BW), and SNR. The control information control field may bereserved.

If the NDP 410 is transmitted based on beamforming, the feedback frame420 may include an HT control field, a channel information controlfield, and a channel information field. The HT control field may includeinformation regarding Nsts, MCS, BW, and SNR. The channel control fieldmay include information regarding Nc, Nr, Ng, etc., and/or controlinformation for OFDMA-based transmission. The channel informationcontrol field may include SNR information per stream, information of abeamforming feedback matrix for each subcarrier, SNR information foreach frequency resource (e.g., subband) in OFDMA-based transmission,etc.

A format of the NDPA frame 400, the NDP 410, and the feedback frame 420is described later in detail.

According to the embodiment of the present invention, upon receiving theNDAP 410, the STA may transmit the feedback frame 420 to the AP after aspecific time (e.g., SIFS). The AP may receive the feedback frame 420,and after the specific time (e.g., SIFS), may transmit a downlink frame430 to the STA on the basis of SU MIMO (or OFDMA/MU-MIMO). The AP maytransmit the downlink frame 430 generated based on channel variationinformation determined by considering channel state information includedin the feedback frame 420.

The STA may transmit an ACK or a block ACK to the AP in response to thereceived downlink frame 430.

If the downlink frame 430 transmitted by the AP receives the feedbackframe 420 and is transmitted after the SIFS, a feedback channel stateparameter (e.g., Nsts, MCS, BW, SNR, Nc, Nr, Ng, SNR information perstream, information of a beamforming feedback matrix for eachsubcarrier, SNR information for each frequency resource (e.g., subband)in OFDMA-based transmission, etc.) is relatively accurate, and thus datatransmission efficiency may be increased.

Hereinafter, FIG. 5 to FIG. 7 disclose the NDPA PPDU and NPD 410 forcarrying the NDPA frame 400 to perform the sounding procedure and thefeedback PPDU for carrying the feedback frame 420 in detail according toan embodiment of the present invention.

FIG. 5 is a concept view illustrating a format of an NDPA PPDU accordingto an embodiment of the present invention.

Referring to an upper part of FIG. 5, the NDPA PPDU may include a PPDUheader and a PSDU (or MPDU). The PPDU header used herein may include aPHY preamble and a PHY header. A PSDU (or MPDU) of the NPDA PPDU mayinclude an NDPA frame.

The PPDU header of the NDPA PPDU may include a legacy-short trainingfield (L-STF) 500, a legacy-long training field (L-LTF) 505, alegacy-signal (L-SIG) 510, a high efficiency-signal A (HE-SIG A) 515, ahigh efficiency-short training field (HE-STF) 520, a highefficiency-long training field (HE-LTF) 525, and a highefficiency-signal B (HE-SIG B) 530. The PHY header may be divided into alegacy part before the L-SIG 510 and a high efficiency (HE) part afterthe L-SIG 510.

The L-STF 500 may include a short training orthogonal frequency divisionmultiplexing (OFDM) symbol. The L-STF 500 may be used for framedetection, automatic gain control (AGC), diversity detection, and coarsefrequency/time synchronization.

The L-LTF 505 may include a long training OFDM symbol. The L-LTF 505 maybe used for fine frequency/time synchronization and channel prediction.

The L-SIG 510 may be used to transmit control information. The L-SIG 510may include information for a data rate and a data length.

The HE-SIG A 515 may include information indicating an STA for receivinga PPDU. More specifically, the HE-SIG A 515 may include informationindicating an STA for receiving an NDPA frame.

In addition, the HE-SIG A 515 may include color bits information for BSSidentification information, bandwidth information, a tail bit, a CRCbit, modulation and coding scheme (MCS) information for the HE-SIG B530, symbol count information for the HE-SIG B 530, and cyclic prefix(CP) (or guard interval (GI)) length information.

The HE-STF 520 may be used to improve automatic gain control estimationin a multiple input multiple output (MIMO) environment or an OFDMAenvironment.

The HE-LTF 525 may be used to estimate a channel in the MIMO environmentor the OFDMA environment.

The HE-SIG B 530 may include a length of physical layer service dataunit (PSDU) for each STA, information regarding modulation and codingscheme (MCS), a tail bit, or the like.

A size of inverse fast Fourier transform (IFFT) applied to the HE-STF520 and a field which comes after the HE-STF 520 may be different from asize of IFFT applied to a field which comes before the HE-STF 520. Forexample, the size of IFFT applied to the HE-STF 520 and the field whichcomes after the HE-STF 520 may be four times greater than the size ofIFFT applied to the field which comes before the HE-STF 520. The STA mayreceive the HE-SIG A 515, and may be instructed to receive a downlinkPPDU on the basis of the HE-SIG A 515. In this case, the STA may performdecoding on the HE-STF 520 and the field which comes after the HE-STF520 on the basis of a changed FFT size. On the contrary, if the STA isnot instructed to receive the downlink PPDU on the basis of the HE-SIG A515, the STA may stop decoding and may configure a network allocationvector (NAV). A cyclic prefix (CP) of the HE-STF 520 may have a sizegreater than a CP of another field, and for this CP duration, the STAmay perform decoding on the downlink PPDU by changing the FFT size.

An order of a field constructing the aforementioned NDPA PPDU format maybe changed.

For example, an HE-SIG B of an HE part may be located immediately afteran HE-SIG A. That is, each field of the HE part may be located in theorder of HE-SIG A, HE-SIG B, HE-STF, HE-LTF. The STA may performdecoding on the HE-SIG and up to the HE-SIG B, may receive necessarycontrol information, and may configure an NAV. Likewise, a size of IFFTapplied to an HE-STF and a field which comes after the HE-STF may bedifferent from a size of IFFT applied to a field which comes before theHE-STF. The STA may receive the HE-SIG A and the HE-SIG B. If it isinstructed to receive the downlink PPDU by the STA identifier field ofthe HE-SIG A, the STA may perform decoding on the downlink PPDU startingfrom the HE-STF by changing the FFT size. On the contrary, the STA mayreceive the HE-SIG A, and if it is not instructed to receive thedownlink PPDU on the basis of the HE-SIG A, may configure the NAV.

A PSDU (or MPDU) 535 of the NDPA PPDU may include the NDPA frame. TheNDPA frame may include a frame control field 540, a duration field 545,an RA field 550, a TA field 555, a sounding dialog token field 560, anSTA information field 565, and an FCS 570.

The frame control field 540 may include a type and subtype forindicating the NDPA frame.

The duration field 545 may include information regarding a duration forprotecting transmission of the NDPA frame.

The RA field 550 may include identification information of an STA forreceiving the NDPA frame. For example, when the STA information fieldincludes information regarding a single STA, the RA field 550 mayinclude address information of the STA. If the STA information field 565includes information regarding a plurality of STAs, the RA field 550 mayinclude a broadcast address of the STA. For example, the RA field 550may include MAC address information of the STA for receiving the NDPAframe.

The TA field 555 may include an address of an AP for transmitting theNDPA frame.

The sounding dialog token field 560 may include information used by theAP for transmitting the NDPA frame to identify the NDPA frame.

The STA information field 565 may include identification information(AID) 575, feedback type information 580, and Nc index information 585of the STA for receiving the NDPA frame.

Table 2 below discloses information included in the STA informationfield.

TABLE 2 Subfield Description AID AID (assoication identifier) of STA forperforming sounding procedure (for receiving NDP to be transmittedlater) is included Feedback type Indicate feedback request type of STAfor performing souding procedure Set to ‘0’ in case of SU-MIMO Set to‘1’ in case of MU-MIMO Nc index Inidicate requested feedback dimension(information regarding the number of columns of beamforming feedbackmatrix) In case of MU-MOMO: Set to ‘0’ if Nc = 1 Set to ‘1’ if Nc = 2Set to ‘2’ if Nc = 3 Set to ‘3’ if Nc = 4 Set to ‘4’ if Nc = 5 Set to‘5’ if Nc = 6 Set to ‘6’ if Nc = 7 Set to ‘7’ if Nc = 8 In case ofSU-MIMO, reserved subfield (Set to 0)

In Table 2, Nc may indicate the number of columns of beamformingfeedback matrices included in a feedback frame transmitted in responseto an NDP.

Upon receiving an NDPA frame, STAs may confirm an AID sub-field valueincluded in an STA information field, and may confirm whether it is asounding target STA.

FIG. 5 is one example for an NDPA PPDU. The NDPA PPDU may have thelegacy PPDU format (e.g., an HT PPDU format, a VHT PPDU format) insteadof the HE-SIG A 515, the HE-STF 520, the HE-LTF 525, and the HE-SIG B530.

FIG. 6 is a concept view illustrating an NDP according to an embodimentof the present invention.

Referring to FIG. 6, the NDP may include only a PPDU header except for aPSDU (or MPDU).

As described above, the NDP (or PPDU header) may include a legacy partand a non-legacy part. Each field included in the legacy part and thenon-legacy part may perform a role described above in FIG. 5 fortransmission of the NDP.

For example, an HE-SIG A 600 may include information indicating an STAfor receiving the NDP.

An HE-LTF 610 may be used for channel estimation of the STA. That is,the STA may perform channel estimation on the basis of the HE-LTF 610included in the NDP frame, and may generate a feedback frame on thebasis of a result of the channel estimation.

An HE-SIG B 620 may include information which indicates a PSDU length of0.

As described above in FIG. 5, an order of some fields included in theaforementioned NDP may be changed. That is, each field of the PPDUheader may be located in the order of HE-SIG A, HE-SIG B, HE-STF, andHE-LTF.

FIG. 7 is a concept view illustrating a feedback PPDU according to anembodiment of the present invention.

Referring to FIG. 7, the feedback PPDU may include a PPDU header and aPSDU (or MPDU). The PSDU (or MPDU) of the feedback PPDU may include afeedback frame.

The PPDU header of the feedback PPDU may include a legacy part and anon-legacy part. Each field included in the legacy part and thenon-legacy part may perform a role described above in FIG. 5 for thefeedback PPDU.

A MAC header of the feedback frame may include an HT control field 700,and the MSDU may include a channel information control field 710 and achannel information field 720. If beamforming is not used in thesounding procedure as described above, the channel information controlfield 710 and the channel information field 720 may not be included inthe feedback frame or may be reserved, and the feedback frame mayinclude only the HT control field 700. Only when the beamforming is usedin the sounding procedure, the feedback frame may include the channelinformation control field 710 and the channel information field 720.

The HT control field 710 may include an MCS feedback (MFB) field. TheMFB field may include information N STS regarding the number ofrecommended space-time streams, information MCS regarding a recommendedMCS index, information BW regarding a bandwidth size intended by arecommended MCS, and information regarding an average SNR measured onall space-time streams and subcarriers for transmitting data.

Table 3 and Table 4 show a format of the channel information controlfield 710 and the channel information field 720.

Table 3 below discloses information included in the channel informationcontrol field 710.

TABLE 3 Subfield Description Nc index Inidicate a value obtained bysubtracting 1 from the number Nc of columns of a beamforming feedbackmatrix Set to 0 if Nc = 1 Set to 1 if Nc = 2 . . . Set to 7 if Nc = 8 Nrindex Inidicate a value obtained by subtracting 1 from the number Nrc ofrows of a beamforming feedback matrix Set to 0 if Nc = 1 Set to 1 if Nc= 2 . . . Set to 7 if Nc = 8 Channel Indicate a size of a channelbandwidth bandwidth measured for generation of a beamforming feedbackmatrix Set to 0 if 20 MHz Set to 1 if 40 MHz Set to 2 if 80 MHz Set to 3if 160 MHz or 80 + 80 MHz Grouping, Ng Inidicate subcarrier groupingused for a beamforming feedback matrix Set to 0 if Ng = 1 (no grouping)Set to 1 if Ng = 2 Set to 2 if Ng = 4 (3 is reserved) Codebook Inidicatea size of codebook entries information Feedback type Inidicate whetherit is beamforming feedback for SU-MIMO or beamforming feedback forMU-MIMO Sounduing Sequency number from NDPA for requesting feedbacksequence

Table 4 below describes information included in the channel informationfield 720.

TABLE 4 Subfield Description SNR(signal to noise ratio) of spatialstream 1 Average SNR on subcarriers in recipient for spatial stream 1 .. . . . . SNR of spatial stream Nc Average SNR on subcarriers inrecipient for spatial stream Nc Beamforming feedback matrix (subcarrierOrder of angles for determining beamforming index 0) feedback matrix forcorresponding subcarrier Beamforming feedback matrix (subcarrier Orderof angles of beamforming feedback index 1) matrix for correspondingsubcarrier . . . . . . Beamforming feedback matrix (subcarrier Order ofangles of beamforming feedback index Ns) matrix for correspondingsubcarrier SNR(signal to noise ratio) of subband 1 Average SNR onsubcarrier included in subband 1 in recipient . . . . . . SNR of subbandNk Average SNR on subcarrier included in subband Nk in recipient

Information of the channel information field 720 disclosed in Table 4may be interpreted on the basis of information included in the channelcontrol field 710 disclosed in Table 3. For example, the AP may receivea feedback frame, and a subcarrier index Ns may be determined on thebasis of channel bandwidth information and grouping information of thechannel information control field 710.

According to another embodiment of the present invention, in thegrouping field Ng, a reserved 3(0×11) may be used for a subband-unitfeedback. If the grouping field Ng is 3, it may indicate thesubband-unit feedback. If Ng=3, an SNR or a beamforming feedback matrix(or a matrix vector) may be fed back in a subband unit. For example, asubband unit for a feedback may be fixed in unit of 26 tones or may beset to an additional unit (e.g., a tone corresponding to a multiple unitof 26 tones) on the basis of an additional subband related field.

In addition, an SNR or a beamforming feedback matrix (or a matrixvector) for a full frequency band may be transmitted through a feedbackPPDU irrespective of whether grouping is achieved.

In addition, an Nc index count may be defined as not being singular(e.g., Nc) but being plural (e.g., Nc₁, Nc₂, etc.), and thus a pluralityof pieces of feedback information corresponding to the plurality of Ncmay be included in the feedback frame.

FIG. 8 is a concept view illustrating a method of transmitting a frameon the basis of a sounding procedure according to an embodiment of thepresent invention.

Disclosed in FIG. 8 is a method of transmitting a frame on the basis ofa sounding procedure for a plurality of STAs. The sounding proceduredisclosed in FIG. 8 is in particular for a case where a DL MUtransmission method and a UL MU transmission method are both supported.

An AP may transmit an NDPA frame 800 to the STA for the soundingprocedure, and may transmit an NDP 810 after a specific time (e.g.,SIFS).

The NDPA frame 800 and the NDP 810 may be transmitted in a non-duplicateformat or a duplicate format in a full transmission band for the NDPAframe 800 and the NDP 800 as described above in FIG. 4.

The NDPA frame 800 and NDP 810 of the non-duplicate PPDU format or theduplicate PPDU format may be transmitted to at least one STA through atleast one stream. If the AP receives a feedback from the plurality ofSTAs as shown in FIG. 8, the AP may transmit the NDPA frame 800 and theNDP 810 to the plurality of STAs through a plurality of space-timestreams. The NDPA frame 800 may indicate the plurality of STAs fortransmitting the feedback frame. The NDP 810 may be used to transmit anLTF through the plurality of space-time streams, and each of theplurality of STAs may transmit to the AP the feedback frame includingchannel state information measured on the basis of the LTF of anindicated space-time stream and an indicated frequency domain.

The NDP 810 may indicate UL MU transmission of the feedback frame by theplurality of STAs. Each of the plurality of STAs may transmit thefeedback frame to the AP after a specific time (e.g., SIFS) from thereception of the NDP 810. Each of the plurality of STAs may receive theNDPA frame 800 and the NDP 810, and may transmit a feedback frame 820 tothe AP on the basis of the UL MU transmission method. For example, eachof the plurality of STAs may transmit the feedback frame on anoverlapping time resource through an allocated space-time stream or afrequency resource.

The AP may receive the feedback frame 820 transmitted based on UL MUtransmission from each of the plurality of STAs. That is, the AP mayreceive each of a plurality of feedback frames transmitted respectivelythrough a plurality of transmission resources respectively allocated tothe plurality of STAs on the overlapping time resource respectively bythe plurality of STAs.

The AP may receive the feedback frame 820 by considering channel stateinformation included in the feedback frame 820, and then, after aspecific time (e.g., SIFS), may transmit a downlink frame 830 (e.g., adownlink data frame, a downlink management frame, etc.) to each of theplurality of STAs on the basis of the DL MU transmission method. Thatis, the AP may transmit a plurality of downlink frames respectively tothe plurality of STAs. Each of the plurality of feedback frames mayinclude channel state information determined on the basis of the NDP810. The plurality of downlink frames may be transmitted respectivelybased on a plurality of pieces of channel state information within aspecific time (e.g., SIFS) after respectively receiving the plurality offeedback frames.

Each of the plurality of STAs may receive the downlink frame 830, andmay transmit an ACK or a block ACK to the AP in response to the downlinkframe 830 on the basis of the UL MU transmission.

Although the downlink frame 830 may be transmitted based on DL MU toeach of all STAs which transmit the feedback frame 820, the downlinkframe 830 may also be transmitted to some STAs among the all STAs whichtransmit the feedback frame 820. For example, the AP may not transmitthe downlink frame 830 for the some STAs among the all STAs whichtransmit the feedback frame 820 by considering the channel stateinformation included in the feedback frame 820. In this case, the AP mayindicate an STA for receiving the downlink frame 830 on the basis of adownlink PPDU for carrying the downlink frame 830. For example, anHE-SIG A field included in a PPDU header of the downlink PPDU mayinclude information indicating the receiving STA. The informationindicating the STA for receiving the downlink PPDU may be an index forindexing STA AIDs included in the NDPA frame 800 in an orderly manner.For example, a first AID may be mapped to an index 0, and a second AIDmay be mapped to an index 1. The STA for receiving the downlink PPDU maybe indicated on the basis of an index value corresponding to the AIDincluded in the downlink PPDU. Alternatively, the STA for receiving thedownlink frame 830 may be indicated on the basis of a group ID.

FIG. 9 is a concept view illustrating a method of transmitting a frameon the basis of a sounding procedure according to an embodiment of thepresent invention.

Disclosed in FIG. 9 is a method of transmitting a frame on the basis ofa sounding procedure for a plurality of STAs. In particular, disclosedis a case where DL MU transmission and UL MU transmission are bothsupported and the number of STAs instructed to transmit a feedback frameis greater than the maximum number of STAs capable of performingtransmission based on UL MU transmission.

An AP may transmit an NDPA frame 900 to the STA for the soundingprocedure, and may transmit an NDP 910 after a specific time (e.g.,SIFS).

The NDPA frame 900 and NDP 910 of the non-duplicate PPDU format or theduplicate PPDU format may be transmitted to at least one STA through atleast one stream. If the AP receives a feedback from the plurality ofSTAs as shown in FIG. 9, the AP may transmit the NDPA frame 900 and theNDP 910 to the plurality of STAs through a plurality of space-timestreams. The NDPA frame 900 may indicate the plurality of STAs fortransmitting the feedback frame. The NDP 910 may be used to transmit anLTF through the plurality of space-time streams, and each of theplurality of STAs may transmit to the AP the feedback frame includingchannel state information measured on the basis of an LTF of anindicated space-time stream and an indicated frequency domain.

The NDPA frame 900 may instruct transmission of feedback frames 920 and940 by using more STAs than the maximum number of STAs capable ofperforming transmission on the basis of UL MU transmission. For example,the number of STAs indicated on the basis of the STA information fieldof the NDPA frame 900 may be greater than the maximum number of STAscapable of performing transmission based on UL MU transmission.

The NDP 910 may indicate UL MU transmission of the feedback frame 920 bythe plurality of STAs. The NDP 910 may be received by the greater numberof STAs than the number of STAs capable of performing transmission basedon UL MU transmission.

Since transmission of the feedback frame is requested to the greaternumber of STAs than the number of STAs capable of performingtransmission based on UL MU transmission, the AP may receive thefeedback frame from the plurality of STAs on the basis of polling. Forexample, if the number of STAs instructed to perform the soundingprocedure on the basis of the NDPA frame 900 is greater than N and themaximum number of STAs capable of performing transmission based on UL MUtransmission is N, N STAs may receive the NDP 910, and after SIFS, maytransmit the feedback frame 920 on the basis of UL MU transmission. Theremaining STAs may transmit the feedback frame 940 on the basis of apolling frame 930 transmitted by the AP.

A transmission priority of the feedback frames 920 and 940 may bedetermined on the basis of an AID of an STA information field includedin the NDPA frame 900. For example, a plurality of STAs corresponding toN AIDs preferentially included among AIDs of the STA information fieldmay preferentially transmit the feedback frame 920 to the AP afterreceiving the NDP 910. STAs corresponding to the remaining AIDs mayreceive the polling frame 930 from the AP, and may transmit the feedbackframe 940 to the AP.

The AP may transmit a downlink frame 950 on the basis of the DL MUtransmission method for all (if the number of STAs capable of performingUL MU transmission is greater than or equal to the number of STAs fortransmitting feedback information) or some (if the number of STAscapable of performing DL MU transmission is less than the number of STAsfor transmitting feedback information) of the plurality of STAs on thebasis of the feedback frames 920 and 940 received from the plurality ofSTAs.

A PPDU header of a downlink PPDU for carrying a downlink frametransmitted based on DL MU transmission may indicate an STA forreceiving a downlink PPDU. Therefore, a plurality of STAs fortransmitting a feedback frame may determine whether to receive (oradditionally decode) the downlink frame 950 on the basis of PPDU headerinformation of the downlink PPDU for carrying the downlink frame 950.

FIG. 10 is a concept view illustrating a method of transmitting a frameon the basis of a sounding procedure according to an embodiment of thepresent invention.

Disclosed in FIG. 10 is a method of transmitting a frame on the basis ofa sounding procedure for a plurality of STAs. In particular, disclosedin FIG. 10 is a case where only a DL MU transmission method issupported.

An AP may transmit an NDPA frame 1000 to the STA for the soundingprocedure, and may transmit an NDP 1010 after a specific time (e.g.,SIFS).

The NDPA frame 1000 and NDP 1010 of the non-duplicate PPDU format or theduplicate PPDU format may be transmitted to at least one STA through atleast one stream. If the AP receives a feedback from the plurality ofSTAs as shown in FIG. 10, the AP may transmit the NDPA frame 1000 andthe NDP 1010 to the plurality of STAs through a plurality of space-timestreams. The NDPA frame 1000 may indicate the plurality of STAs fortransmitting the feedback frame. The NDP 1010 may be used to transmit anLTF through the plurality of space-time streams, and each of theplurality of STAs may transmit to the AP the feedback frame includingchannel state information measured on the basis of an LTF of anindicated space-time stream and an indicated frequency domain. The NDP1010 may instruct transmission of a feedback frame 1015 of a single STAamong a plurality of STAs indicated based on the NDPA frame 1000. Forexample, an STA corresponding to a first AID among AIDs included in theSTA information field of the NDPA frame 1000 may receive the NDP 1010,and after a specific time (e.g., SIFS), may transmit the feedback frameto the AP.

At least one STA corresponding to the remaining AIDs may sequentiallyreceive polling frames 1020 and 1030 from the AP, and may transmitfeedback frames 1015, 1025, or the like to the AP.

The AP may receive the feedback frames 1015, 1025, or the like from theplurality of STAs, and may determine channel variation information andtransmit a downlink frame 1040 on the basis of the DL MU transmissionmethod. The plurality of STAs for receiving the downlink frame 1040transmitted based on the DL MU transmission method may be all or someparts of the plurality of STAs for transmitting the feedback frames1015, 1025, or the like.

FIG. 11 and FIG. 12 disclose NDPA PPDU and NPD formats for carrying anNDPA frame to perform a sounding procedure in detail according to anembodiment of the present invention.

FIG. 11 is a concept view illustrating an NDPA PPDU format according toan embodiment of the present invention.

Disclosed in FIG. 11 is an NDPA PPDU transmitted to each of a pluralityof STAs through a different downlink transmission resource (a frequencyresource or a spatial stream). The NDPA frame transmitted through adifferent space-time stream or a different frequency resource mayinclude different information.

The NDPA PPDU may be transmitted to a plurality of STAs through a fullbandwidth allocated on the basis of a non-duplicate format, or may betransmitted to the plurality of STAs through a plurality of channels onthe basis of a duplicate format. In this case, the aforementioned NDPAPPDU format of FIG. 5 is used, and an HE-SIG A field may indicate theplurality of STAs.

Referring to FIG. 11, a field which comes before an HE-SIG B 1110 on theNDPA PPDU may be transmitted in a duplicated form in each of differentuplink transmission resources. The HE-SIG B 1110 may be transmitted onall transmission resources in an encoded form. A field which comes afterthe HE-SIG B 1110 may include individual information for each of theplurality of STAs for receiving the PPDU. For example, a field whichcomes after the HE-SIG B 1110 to be transmitted through a firsttransmission resource may include an STA information field indicating anSTA1, and a field which comes after the HE-SIG B 1110 to be transmittedthrough a second transmission resource may include an STA informationfield indicating an STA2.

If the field included in the NDPA PPDU is transmitted through each ofthe downlink transmission resources, a CRC for each field may beincluded in the NDPA PPDU. On the contrary, if a specific field includedin the NDPA PPDU is transmitted by being encoded on all downlinktransmission resources, a CRC for each field may not be included in theNDPA PPDU. Therefore, an overhead for the CRC may be decreased. The NDPAPPDU may include the HE-SIG B 1110 having an encoded form on alltransmission resources. When using the HE-SIG B 1110 in this format, aCRC overhead may be decreased.

An HE-SIG A 1100 of the NDPA PPDU may include information indicating aplurality of STAs for receiving the NDPA PPDU and/or informationregarding transmission resources allocated respectively to the pluralitySTAs to receive respective NDPA PPDUs of the plurality of STAs. That is,the HE-SIG A 1100 may include information regarding the transmissionresources allocated respectively to the plurality of STAs. Each of theplurality of STAs may receive the NDPA PPDU through a transmissionresource indicated based on the HE-SIG A 1100.

Likewise, also in a NDPA PPDU format for DL MU transmission, an HE-STF1120 and a field which comes after the HE-STF 1120 may be encoded on thebasis of an IFFT size different from that of a field which comes beforethe HE-STF 1120. Therefore, the STA may receive an HE-SIG A 1100 and theHE-SIG B 1110, and if it is instructed to receive the NDPA PPDU on thebasis of the HE-SIG A 1100, may perform decoding on the NDPA PPDU bychanging an FFT size.

The downlink frame (downlink data frame, downlink management frame)transmitted based on UL MU transmission disclosed in the embodiment ofthe present invention may also be transmitted to the plurality of STAson the basis of the same PPDU format as the NDPA PPDU format disclosedin FIG. 11.

FIG. 12 is a concept view illustrating an NDP according to an embodimentof the present invention.

Referring to an upper portion of FIG. 12, the NDP may include only aPPDU header except for a PSDU (or MPDU).

Disclosed in FIG. 12 is an NDP transmitted to each of a plurality ofSTAs through a different downlink transmission resource (a frequencyresource or a spatial stream). The NDP transmitted through a differentspace-time stream or a different frequency resource may includedifferent information.

As described above, the NDP (or PPDU header) may include a legacy partand a non-legacy part. Each field included in the legacy part and thenon-legacy part may perform a role described above in FIG. 5.

A field which comes before an HE-SIG B 1210 in the NDP may betransmitted in a duplicated form in each of different uplinktransmission resources. The HE-SIG B 1210 may be transmitted on alltransmission resources in an encoded form. A field which comes after theHE-SIG B 1210 may include individual information for each of theplurality of STAs for receiving the PPDU.

For example, an HE-SIG A 1200 may include information indicating aplurality of STAs for receiving the NDP and information regardingtransmission resources allocated respectively to the plurality of STAs.Each of the plurality of STAs may receive the NDP through a transmissionresource indicated based on the HE-SIG A.

Each of the plurality of STAs may receive an HE-LTF transmitted throughthe transmission resource indicated based on the indicated HE-SIG A1200, and may perform channel estimation to generate feedbackinformation.

FIG. 13 is a concept view illustrating a PPDU format for a soundingprocedure according to an embodiment of the present invention.

Disclosed in FIG. 13 is a sounding procedure based on a single PPDU.

Referring to FIG. 13, a new sounding PPDU may be defined in which theNDPA PPDU is followed by a training field (e.g., an HE LTF2 1300) addedthereto for channel estimation. The new sounding PPDU in which the NDPAPPDU is followed by the training field 1300 added thereto for channelestimation may be defined by the term ‘synthesized sounding PPDU’. Ifthe synthesized sounding PPDU is used, additional NDP transmission maynot be performed.

Alternatively, after the synthesized sounding PPDU is transmitted tosecure a processing time for generating a feedback frame of the STA, theNDP may be transmitted with a format of a null frame not including thetraining field (e.g., HE-LTF). The STA may not perform additionaldecoding on the NDP, may generate channel state information based on thesynthesized sounding PPDU, and may transmit the generated channel stateinformation by including it to the feedback frame.

FIG. 14 is a concept view illustrating a PPDU format for carrying an ACKframe according to an embodiment of the present invention.

Disclosed in FIG. 14 is a PPDU for carrying an ACK frame based on UL MU.The ACK frame based on UL MU disclosed in FIG. 14 may be used totransmit an ACK or a block ACK based on UL MU of a plurality of STAs asdescribed above.

Although it is assumed in FIG. 14 that a subband granularity is 5 MHz, aPPDU format for carrying the same ACK frame may be used by consideringexpansion or reduction on a frequency band also in a subband granularityof 20 MHz and 2.5 MHz. Respective subbands may be frequency resourcesthat can be allocated respectively to STAs for OFDMA-based transmission.

Referring to FIG. 14, a frequency band of 20 MHz may include four 5 MHzsubbands. Each of the four 5 MHz subbands may be used for OFDMA-based DLMU transmission to the plurality of STAs.

A UL MU ACK frame may be transmitted in response to a plurality ofdownlink PPDUs transmitted based on DL MU transmission.

A PPDU (UL MU ACK PPDU) for carrying a UL MU ACK frame may include an HEpart. The HE part may include an HE-STF 1400, an HE-SIG field 1410, andan HE-ACK data field 1420. Alternatively, the UL MU ACK PPDU may includethe HE part (the HE-STF 1400, the HE-SIG field 1410) as a PPDU header,and may include an ACK frame 1420 as a PSDU (or MPDU).

An OFDM symbol for transmission of each field included in the UL MU ACKPPDU may include an extended CP. Further, the UL MU ACK PPDU may includethe legacy part (L-STF, L-LTF, and L-SIG), but may also have a formatnot including the legacy part as shown in FIG. 14. A length of theextended CP may be two times or four times longer than a length of a CPof an OFDM symbol for transmission of the HE-STF and HE-SIG included ina downlink PPDU transmitted through DL MU. By using a CP having a longlength, a reception timing error may be compensated for between aplurality of uplink transmissions based on UL MU.

In a UL MU ACK PPDU, an HE-STF and an HE-SIG may be transmitted throughan overlapping frequency resource (or a frequency band including allsubbands) on an overlapping time resource, and the remaining ACK data(or ACK frames) may be transmitted through a frequency resourceallocated for each STA. That is, the AP may receive a plurality ofpieces of ACK data respectively for a plurality of downlink framesrespectively through a plurality of transmission resources allocatedrespectively to a plurality of STAs on an overlapping time resource fromthe plurality of STAs. In this case, a training field and signal fieldfor decoding of each of the plurality of pieces of ACK data may betransmitted through a plurality of transmission resources.

For example, referring to the UL MU ACK PPDU format 1, when assumingfour subbands having a subband granularity of 5 MHz, each of an STA1 toan STA4 may transmit an HE-STF and an HE-SIG through a 20 MHz band whichis a frequency band including a full subband, and may transmit ACK datathrough respective 5 MHz subbands allocated thereto.

Alternatively, in the UL MU ACK PPDU, the HE-STF, HE-SIG, and ACK data(or ACK frame) may be transmitted through an allocated frequencyresource. For example, referring to the UL MU ACK PPDU format 2, whenassuming four subbands having a subband granularity of 5 MHz, each ofthe STA1 to the STA4 may transmit the HE-STF, the HE-SIG, and the ACKdata (or ACK frame) through the respective 5 MHz subbands allocatedthereto.

This method may also be applied to a case where a downlink frame istransmitted and a UL MU ACK PPDU is transmitted based on DL MU MIMOtransmission. The UL MU ACK PPDU transmitted in response to a downlinkframe on the basis of DL MU MIMO transmission may be transmitted on thesame bandwidth as a transmission bandwidth of the downlink frame. Forexample, if the downlink frame is transmitted based on DL MU MIMOtransmission on 20 MHz, each of the plurality of STAs may transmit anACK PPDU (or ACK frame) on the basis of UL MU MIMO transmission on 20MHz.

In order for the plurality of STAs to transmit an ACK PPDU on the basisof UL MU MIMO transmission, an SIG field or control field of a downlinkPPDU for carrying a downlink frame on the basis of DL MU MIMOtransmission may include information for UL MU MIMO transmission. Forexample, information for the UL MU MIMO transmission may includeresource allocation information (e.g., space-time stream indicationinformation) used by each of the plurality of STAs to transmit an ACKframe.

Alternatively, the UL MU MIMO method to be used for transmission of theACK frame of the plurality of STAs may be determined based on the DL MUMIMO method used for transmission of a downlink frame or may bedetermined by considering a predetermined (or agreed) method (e.g., amethod determined in advance by an agreement or information signaled bya beacon frame).

FIG. 15 is a block diagram of a wireless device according to anembodiment of the present invention.

Referring to FIG. 15, a wireless device 1500 is an STA capable ofimplementing the aforementioned embodiment, and may be an AP 1500 or anon-AP STA (or STA) 1550.

The AP 1500 includes a processor 1510, a memory 1520, and a radiofrequency (RF) unit 1530.

The RF unit 1530 may be coupled to the processor 1510 totransmit/receive a radio signal.

The processor 1510 may implement the functions, procedures, and/ormethods proposed in the present invention. For example, the processor1510 may be implemented to perform an operation of an AP according tothe embodiment of the present invention described above. The processormay perform the operation of the AP in the embodiment of the FIG. 1 toFIG. 14.

For example, the processor 1510 may be implemented to transmit an NDPAframe (or each of a plurality of NDPA frames) to each of a plurality ofSTAs and to transmit an NDP (or each of a plurality of NDPs) to each ofthe plurality of STAs. The NDPA frame (or each of the plurality of NDPAframes) may report transmission of the NDP (or each of the plurality ofNDPs).

Further, the processor 1510 may be implemented to receive a plurality offeedback frames transmitted respectively through a plurality oftransmission resources allocated respectively to the plurality of STAson an overlapping time resource respectively by the plurality of STAs,and to transmit a plurality of downlink frames respectively to theplurality of STAs. The plurality of feedback frames respectively includea plurality of pieces of channel state information determined on thebasis of the NDP (or the plurality of NDPs respectively). The pluralityof downlink frames may respectively receive the plurality of feedbackframes and thereafter may be transmitted based on the plurality ofrespective pieces of channel state information within a specific time.

The STA 1550 includes a processor 1560, a memory 1570, and a radiofrequency (RF) unit 1580.

The RF unit 1580 may be coupled with the processor 1560 totransmit/receive a radio signal.

The processor 1560 may implement the functions, procedures, and/ormethods proposed in the present invention. For example, the processor1560 may be implemented to perform an operation of an STA according tothe embodiment of the present invention described above. The processormay perform the operation of the STA in the embodiment of the FIG. 1 toFIG. 14.

For example, by the use of the processor 1560, the STA may receive theNDP from the AP to perform channel estimation, and may transmit afeedback frame to the AP on the basis of a UL MU transmission method.

The processors 1510 and 1560 may include an application-specificintegrated circuit (ASIC), a separate chipset, a logic circuit, a dataprocessing unit, and/or a converter for mutually converting a basebandsignal and a radio signal. The memories 1520 and 1570 may include aread-only memory (ROM), a random access memory (RAM), a flash memory, amemory card, a storage medium, and/or other equivalent storage devices.The RF units 1530 and 1580 may include one or more antennas fortransmitting and/or receiving the radio signal.

When the embodiment of the present invention is implemented in software,the aforementioned methods can be implemented with a module (i.e.,process, function, etc.) for performing the aforementioned functions.The module may be stored in the memories 1520 and 1570, and may beperformed by the processors 1510 and 1560. The memories 1520 and 1570may be located inside or outside the processors 1510 and 1560, and maybe coupled to the processors 1510 and 1560 by using various well-knownmeans.

1-8. (canceled)
 9. A method of transmitting a frame based on a sounding procedure in a wireless local area network (WLAN), the method comprising: transmitting, by a transmitting station, a null data packet announcement (NDPA) frame; transmitting, by the transmitting station, a null data packet (NDP) frame following the NDPA frame; receiving, by the transmitting station, a first group of feedback frames from a first group of reporting stations, wherein the first group of feedback frames are received based on an uplink multi-user operation, which enables the first group of feedback frames to be received by the transmitting station simultaneously; after receiving the first group of feedback frames, delaying, by the transmitting station, during a short inter-frame space (SIFS), and transmitting, by the transmitting station, a polling frame to a second group of reporting stations immediately after the SIFS; and receiving, by the transmitting station, a second group of feedback frames from the second group of reporting stations, wherein the second group of feedback frames are received based on the uplink multi-user operation, which enables the second group of feedback frames to be received by the transmitting station simultaneously.
 10. The method of claim 9, wherein frequency resources for the first group of feedback frames are separately allocated by the transmitting station.
 11. The method of claim 9, wherein the polling frame indicates frequency resources allocated for the second group of feedback frames and identifiers of the second group of reporting stations.
 12. The method of claim 9, wherein the NDP frame comprises a first physical layer protocol data unit (PPDU) part including a legacy training field and a legacy signal field and a second PPDU part including a high efficiency short training field (HE-STF) and a high efficiency long training field (HE-LTF) and does not comprise a protocol service data unit (PSDU) field, wherein the first PPDU part is configured by a first point fast fourier transform (FFT) operation and the second PPDU part is configured by a second point FFT operation such that a subcarrier spacing of the second PPDU part is four times fewer than a subcarrier spacing of the first PPDU part.
 13. A transmitting station for transmitting a frame based on a sounding procedure in a wireless local area network (WLAN), the transmitting station comprising: a radio frequency (RF) unit implemented to transmit or receive a radio signal; and a processor operatively coupled to the RF unit, wherein the processor is configured to: transmit a null data packet announcement (NDPA) frame; transmit a null data packet (NDP) frame following the NDPA frame; receive a first group of feedback frames from a first group of reporting stations, wherein the first group of feedback frames are received based on an uplink multi-user operation, which enables the first group of feedback frames to be received by the transmitting station simultaneously; after receiving the first group of feedback frames, delay during a short inter-frame space (SIFS), and transmit a polling frame to a second group of reporting stations immediately after the SIFS; and receive a second group of feedback frames from the second group of reporting stations, wherein the second group of feedback frames are received based on the uplink multi-user operation, which enables the second group of feedback frames to be received by the transmitting station simultaneously.
 14. The transmitting station of claim 13, wherein frequency resources for the first group of feedback frames are separately allocated by the transmitting station.
 15. The transmitting station of claim 13, wherein the polling frame indicates frequency resources allocated for the second group of feedback frames and identifiers of the second group of reporting stations.
 16. The transmitting station of claim 13, wherein the NDP frame comprises a first physical layer protocol data unit (PPDU) part including a legacy training field and a legacy signal field and a second PPDU part including a high efficiency short training field (HE-STF) and a high efficiency long training field (HE-LTF) and does not comprise a protocol service data unit (PSDU) field, wherein the first PPDU part is configured by a first point fast fourier transform (FFT) operation and the second PPDU part is configured by a second point FFT operation such that a subcarrier spacing of the second PPDU part is four times fewer than a subcarrier spacing of the first PPDU part. 